Metallic bearings for joint replacement

ABSTRACT

An orthopedic prosthesis comprising a first component having a soft metal bearing surface and a second component having a hard metal bearing surface, in particular a hip prosthesis comprising a spherical bearing member, a femoral stem, and a modular neck configured for interposition between the spherical bearing member and the stem, together with an acetabular implant having a concave bearing surface configured for articulation with the spherical bearing member. One of the spherical bearing member and the concave bearing surface comprises a soft metal bearing surface, the soft metal bearing surface having a hardness of at least about 20 Rc, while the other one of the spherical bearing member and the concave bearing surface comprises a hard metal bearing surface, the hard metal bearing surface having a hardness greater than the soft metal bearing surface by at least about 15 Rc.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S. Pat.App. Ser. No. 10/965,491, now U.S. Pat. No. 7,361,194 filed Oct. 14,2004.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable

REFERENCE TO A MICROFICHE APPENDIX

Not applicable

FIELD OF THE INVENTION

The present invention relates to orthopedic prostheses, and moreparticularly to prostheses having differential hardness metal bearingsin order to decrease wear debris and increase the useful life of theprostheses.

BACKGROUND OF THE INVENTION

Orthopedic joint prostheses are used to replace diseased joints, such asin the hip, knee, ankle or shoulder. An orthopedic joint prosthesisincludes bearing surfaces that allow for articulation similar to thatprovided by the articulating surfaces of a natural joint.

One problem associated with orthopedic joint prostheses is wear of thebearing components. During articulation, the bearing surfaces slideagainst each other under load, which results in wear of the bearingsurfaces, including loss of minute particles from the bearing surfaces.Over time, such particulates accumulate in the body of the patient,where it is theorized that the particles may cause adverse physiologicalreactions in some patients. Additionally, gradual loss of particulatesresults in erosion of the bearing surfaces, which may eventually lead tofailure of the prosthesis. Various efforts have been made to minimizewear debris in joint prostheses. In recent years, efforts have focusedon the use of metal-on-metal (“MOM”) joint prostheses, in which both ofthe articulating surfaces are metal, and ceramic-on-metal (“COM”)prostheses, in which one of the articulating surfaces is ceramic and theopposing surface is metal, and ceramic-on-ceramic (“COC”) prostheses, inwhich both of the articulating surfaces are ceramic.

Lower wear is expected with COC and COM combinations due to theincreased abrasion resistance provided by the hard ceramic materials.Differences in material stiffness, especially in COM combinations, havebeen shown to facilitate bearing lubrication with resulting decreasedwear. Additionally, COM bearing combinations have a relatively highhardness differential ranging from about 3× to about 5×, and typicallyon the order of 4×, which is thought to contribute to lower wear rates.

Despite the low wear properties of COM and COC prostheses, ceramic headspresent a risk of fracture, a risk that may steer some surgeons awayfrom COM prostheses, despite the benefits of low wear. In addition, thebrittleness and lower toughness of ceramic materials make it difficultto manufacture large diameter femoral heads of the type used inresurfacing procedures. MOM prostheses produce less wear particles thanconventional metal-on-polymer (“MOP”) prostheses. MOM joint prosthesesare typically made of cobalt based alloys conforming to ASTM F75 and/orF1537 specifications. In conventional MOM joint prostheses, the opposingbearing surfaces are made of the same cobalt chrome alloy and thereforehave substantially the same hardness (typically, ranging from about 25to 45 Rc). Thus far, little attention has been paid to the hardness ofthe MOM bearing surfaces. Factors other than hardness were thought tohave a greater effect on wear rate in MOM hip prostheses. The mostimportant wear factors are surface finish, clearance, and sphericity.Recent efforts to improve the performance of MOM joint prostheses havetherefore focused on improving surface finish, clearance, andsphericity, rather than on the hardnesses of the cobalt chrome bearingsurfaces.

Although MOM prostheses have lower wear volumes than MOP prostheses,metal wear particles are very small and high in number, and thephysiological effect of metal wear particles is not fully understood.There is thus an interest in further reducing the volume of wearparticles from MOM prostheses.

In ceramic-on-metal (“COM”) joint prostheses, the ceramic bearingsurface is significantly harder than the metal bearing surface, andtherefore necessarily provides a hybrid bearing effect. WO 01/17464A1(Fisher et al.) discusses improved wear in orthopedic prostheses throughthe use of COM. WO 01/17464A1 disclosed that materials of the twosurfaces can be selected with hardnesses that are greater than those inother joint systems so that the tendency for them to wear duringarticulation is reduced, and with a differential hardness which canensure that one of the surfaces is generally able to remain smoothduring articulation. This in turn can result in low wear of the oppositesurface. WO 01/17464A1 noted that the use of a ceramic material that issignificantly harder than the metal material has the advantage that thetendency of the ceramic material to wear during articulation isminimized. WO 01/17464 included wear testing data showing that weardebris from a COM prosthesis was significantly less than from a MOMprosthesis. According to WO 01/17464, the MOM prosthesis showed abedding-in wear rate of 3.12±0.45 mm³/10⁶ cycles for about the firstmillion cycles, which settled down to a steady state wear rate of1.56±0.78 mm³/10⁶ cycles. In contrast, the COM prosthesis showedessentially no bedding-in phase and a steady state rate of about 0.01mm³/10⁶ cycles over the course of a 3 million cycle test. Substantiallyall of the wear debris from the COM components was metal.

Further data comparing COM and MOM hip prostheses is provided in A NovelLow Wearing Differential Hardness, Ceramic-On-Metal Hip JointProsthesis, 34 J'l of Biomechanics, 1291-1298 (2001) (Firkins et al.).The Firkins article reported wear rates for MOM prostheses that showed a100 fold higher degree of wear than for COM prostheses. Id. at 1296.Firkins tested femoral heads manufactured from medical grade alumina(ISO 6474) and femoral heads manufactured from medical grade low carbon(less than 0.07 percent) wrought cobalt chrome alloy (ASTM F1537). Id.at 1293. Firkins coupled the ceramic and cobalt chrome heads withacetabular cups manufactured from medical grade high carbon (greaterthan 0.2 percent) wrought cobalt chrome alloy (ASTM F1537). Id. at 1293.Firkins reported a bedding in rate for MOM prostheses of 3.09±0.46mm³/10⁶ cycles during the first million cycles and a steady state wearrate of 1.23±0.5 mm³/10⁶ cycles. Id. at 1294. The overall wear rate forMOM prostheses during the test was 1.62 mm³/10⁶ cycles. Id. at 1294.Firkins noted that about 70 percent of the wear on the MOM prosthesesoccurred on the low carbon cobalt chrome heads. Id. at 1294. In contrastwith MOM prosthesis wear, Firkins reported a wear rate on the COMprostheses of 0.1 mm³/10⁶ cycles during a five million cycle test. Id.at 1294. The results of Firkins thus suggest that MOM prostheses wear ata rate 100 times greater than that of COM prostheses. Id. at 1294-96.

European Patent Application 841 041 A2 (Farrar) reported improved wearif the two articulating surfaces of a metal-on-metal prosthesis areformed from metals which are mismatched with respect to their carboncontent. According to EP 841 041A2, testing of MOM hip prostheses havingmismatched carbon contents demonstrated that the lowest average wear(weight loss) was observed for prostheses in which a low carbon contentalloy was used for the femoral head and a high carbon content alloy wasused for the acetabular cup, or vice versa. (Col. 4, lines 46-53). EP841 041 A2 reported that the highest average wear was observed forprostheses in which the femoral head and acetabular cup were both formedfrom low carbon content alloy or both formed from high carbon contentalloy. (Col. 4, lines 46-53). A1 though EP 841 041 A2 reported testingof MOM prostheses for up to two million cycles, the patent failed toreport actual wear test data, and it therefore impossible toquantitatively evaluate the wear claims made in the patent.

EP 841 041 A2 did not note a difference in hardness between themismatched carbons. This is probably because CoCrMo alloys with lowcarbon contents of different carbon contents have essentially the samehardness values. For example, a CoCr with a low carbon content of 0.07percent-by-weight can have a hardness of 41 Rc, while a CoCr with a highcarbon content of 0.25 percent-by-weight can have a hardness of 42 Rc.

In the general field of metal bearings, hybrid bearings consisting of ahard metal bearing in combination with a soft metal bearing have longbeen used to reduce wear of the bearing components. However, thehardness differential of hybrid bearings is provided by forming thebearing surfaces from two different types of metal, rather than from thesame type of metal.

Despite prior art COM prostheses that by default have differentialhardness bearing surfaces due to the use of different materials for thetwo bearing surfaces, no attempt has been made to explore the extent towhich differential hardness concepts apply to the wear of MOMprostheses. There is thus a need for a joint prosthesis having thefollowing characteristics and advantages over the prior art.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the invention to provide an MOM orthopedic jointprosthesis that reduces wear through the use of differential hardness.

It is another object of the invention to provide a MOM orthopedic jointprosthesis that reduces the volume of wear debris in a recipient of theprosthesis through the use of cobalt chrome alloys that are currentlyused for joint prostheses.

These and other objects of the invention are achieved by providing anorthopedic joint prosthesis comprising a first component having a softmetal bearing surface and a second component having a hard metal bearingsurface. The soft metal bearing surface has a hardness of at least about20 Rc, while the hard metal bearing surface having a hardness greaterthan the soft metal bearing surface by at least about 15 Rc. The softand the hard metal bearing surfaces are configured to articulate withone another. The differential hardness of the hard metal bearing surfaceto the soft metal bearing surface is preferably at least about 1.5, andis preferably less than about 3. The hard metal bearing surface ispreferably not more than about 40 Rc harder than the soft metal bearingsurface. The hard metal bearing surface preferably has a hardness ofbetween about 40 Rc and about 60 Rc.

In one preferred embodiment, the concepts of the invention are used in ahip prosthesis comprising a spherical bearing member, the sphericalbearing member having a neck recess therein; a femoral stem, the femoralstem having a recess adjacent a proximal end thereof, a modular neckmember, a proximal end of the modular neck member configured to fixedlyengage the neck recess of the spherical bearing member, a distal end ofthe modular neck member configured to fixedly engage the recess of thestem; and an acetabular implant having a concave bearing surfaceconfigured for articulation with the spherical bearing member. One ofthe spherical bearing member and the concave bearing surface comprises asoft metal bearing surface of cobalt chrome alloy, the soft metalbearing surface having a hardness of at least about 20 Rc, while theother one of the spherical bearing member and the concave bearingsurface comprises a hard metal bearing surface of cobalt chrome alloy,the hard metal bearing surface having a hardness greater than the softmetal bearing surface by at least about 15 Rc, such that there is ahardness differential between the spherical member and the concavebearing surface to thereby reduce wear of the hip prosthesis. In onepreferred embodiment, the spherical bearing member has a diameter ofbetween about 40 mm to about 60 mm, such as 54 mm.

The foregoing and other objects, features, aspects and advantages of theinvention will become more apparent from the following detaileddescription of the invention when considered in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is representative view of an orthopedic prosthesis incorporatingthe metal-on-metal differential hardness concepts of the presentinvention.

FIG. 2 is a graph demonstrating the wear of a differential hardness MOMhip prosthesis according to the invention in comparison with wear forconventional MOM and COM hip prostheses.

FIG. 3 is a graph demonstrating the wear rates of a differentialhardness MOM hip prosthesis according to the invention in comparisonwith wear rates for a conventional MOM prosthesis of equal hardness.

FIG. 4 shows a side view of a modular neck hip prosthesis structure thatcan be used with the differential hardness concept of the presentinvention.

FIG. 5 shows a front view of one embodiment of a modular neck hipprosthesis stem.

FIG. 6 is a cross-sectional view illustrating a coupling between thestem and a coupling member/modular neck of the prosthesis.

FIG. 7 shows a possible embodiment of the prosthesis couplingmember/modular neck.

FIG. 8 shows one embodiment of a straight modular neck.

FIG. 9 shows one embodiment of a varus modular neck.

FIG. 10 shows one embodiment of a retroverted modular neck.

FIG. 11 shows one embodiment of an anteverted modular neck.

FIG. 12 shows one embodiment of a lateralized/medialized offset modularneck.

FIG. 13 shows one embodiment of a prosthesis stem configured for receiptof a coupling member;

FIG. 14 is an end view of a coupling member to be used with theprosthesis stem illustrated in FIG. 13.

FIG. 15 is a cross-sectional view of a coupling member to be used withthe prosthesis stem illustrated in FIG. 13.

PREFERRED EMBODIMENTS OF THE INVENTION

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings which form a part hereof,and in which are shown by way of illustration specific embodiments inwhich the invention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

The invention is an orthopedic joint prosthesis in which the material ofone of the bearing surfaces is a metal material and the material of theother bearing surface is a metal material having a different hardnessthan that of the other bearing surface. For the sake of consistency andclarity in the discussion of this invention, the bearing surface havingthe softer metal will be referred to as the “soft metal bearingsurface,” while the bearing surface comprising the harder metal will bereferred to as the “hard metal bearing surface.” Of course “soft” and“hard” metal bearing surface are relative terms, and they are usedherein to describe a hardness relationship between the bearing surfaces,rather than an absolute physical property.

FIG. 1 provides a representative view of a hip prosthesis incorporatingthe differential hardness metal bearing concept of the invention. Theprosthesis includes a first component 30 having a first bearing surface32, and a second component 50 having a second bearing surface 52. Thebearing surfaces 32, 52 are sized and configured to articulate relativeto one another in the manner of a natural joint. For example, in FIG. 1,the first bearing surface 32 is convex while the second bearing surface52 is concave, such that the first 32 and second 52 bearing surfacesarticulate relative to one another in the manner of a hip or shoulderprosthesis. In a preferred embodiment, the concave bearing surface is asoft metal bearing surface and the convex bearing surface is a hardmetal bearing surface. However, the concave bearing surface may be thehard metal bearing surface and the convex bearing surface may be thesoft metal bearing surface.

The soft metal bearing surface preferably has a hardness of betweenabout 25 Rc to about 35 Rc. The hard metal bearing surface preferablyhas a hardness of between about 40 to about 60 Rc.

Cobalt chrome alloys are the preferred metal for use in the invention.Cobalt chrome has good wear properties and can be accurately configuredto provide desired sphericity, clearance and surface finish. Thedifferential hardness concept of the invention can be applied to allmedical grade CoCrMo alloys that have been approved by the FDA formedical devices. The soft metal bearing surface preferably has a carboncontent of 0.2 to 0.25 percent-by-weight. The hard metal bearing surfacepreferably has a carbon content of 0.2 to 0.3 percent-by-weight, but lowcarbon alloys (less than 0.10 percent-by-weight) may be used. Thedifferential hardness concept of the invention can be applied to CoCrMoalloys that have little or no carbon content.

The preferred diameters of the bearing surfaces will match those ofexisting prostheses. In the case of hip prostheses, the preferreddiameter will generally fall between about 20 to 60 mm, with preferredranges further varying depending on the type of hip prosthesis.

The first and second components of the prosthesis of the invention canbe made entirely of the metal that provides the bearing surfaces.Alternatively, in one or both of the components, the material of thebearing surfaces may provide only a portion of the components. Forexample, the spherical head of the femoral component of a hip prosthesismay be formed of cobalt chrome, with the spherical head having a taperedhollow for receiving a tapered pin of the main body part of the femoralcomponent. The acetabular component can be manufactured as a separateshell and insert, with the insert providing the bearing surface.

The differential hardness concept of the invention can be applied to amodular hip prosthesis of the type described in U.S. Pat. No. 4,957,510(Cremescoli) and its European counterpart, EP0310566B1, both of whichare incorporated herein by reference. Thus, the prosthesis could beprovided with a spherical head configured to engage an end of a separateneck member.

An example of a modular neck embodiment is shown in FIGS. 4-15, whichare also shown in U.S. Pat. No. 4,957,510. With reference to FIGS. 4-15,the hip prosthesis structure essentially comprises a stem 1 consistingof a flat bar having a given length. The surface of the stem 1 may beprovided with a plurality of longitudinal slots 2. In the embodimentshown in FIG. 4, the stem 1 extends according to a suitably curvedlongitudinal axis and has a bottom end 3 of substantially oval shape.The stem 1 is provided, at the top thereof, with a portion 4 enlarged atthe side of its concave perimetrical line on which there is formed aseat or recess 5 having a suitably slanted axis, an oval cross-section,and suitable taper. As shown in FIG. 5, the seat 5 preferably has a“race track” configuration in cross-section, with opposing flat sidesand opposing curved portions. As shown in FIGS. 4-6, the enlargedportion 4 may have a grooved slanted face in which there are formed aplurality (three in the shown embodiment) of spaced grooves which havebeen specifically designed for receiving bone in-growth so as toprovide, in cooperation with the slots 2, a very firm gripping of thestem 1 in the femur (not shown). As indicated in FIGS. 4 and 6, the seat5 is configured to firmly house a stem end 6 of a coupling member ormodular neck 7. The stem end 6 is also of oval cross-section. A femoralhead end 8 of the modular neck 7 has a frustum of cone shape or Morsetaper to firmly restrain a spherical head 9 (corresponding to bearing30) adapted in turn for coupling with the acetabulum of the pelvis ofthe patient. The modular neck 7 may have any desired variable length,depending on the specific use requirements.

The modular neck 7 may have a differently slanted axis with respect toits stem end 6, with the axis of the stem end 6 coinciding with the axisof the seat 5. This slanting can be essentially obtained according toany of the planes pertaining to the plane set passing through the linedefining the axis of the seat 5. FIGS. 8-12 show examples of possibleconfigurations of modular necks 7 adapted to be applied on theprosthesis stem 1 and having different extension longitudinal axes. FIG.8 shows a straight modular neck 7 in which the axis of the femoral headend 8 is coincident with the axis of the seat 5. FIG. 9 shows a varusmodular neck 7 in which the axis of the femoral head is offset towardthe midline from the axis of the seat 5. FIG. 10 shows a retrovertedmodular neck in which the axis of the femoral head 8 is offsetposteriorly from the axis of the seat 5. FIG. 11 shows an antevertedmodular neck in which the axis of the femoral head 8 is offsetanteriorly from the axis of the seat 5. As is shown in FIG. 12, anembodiment of the present invention provides for the use of a modularneck/coupling member 7 including an insertion end 6 which is offset fromits axial extension 8. Thus, a prosthesis will be formed including acoupling member 7 which virtually constitutes an extension of the middlecurvature, indicated at 10, of the top portion of the stem 1. Thisconfiguration will afford the possibility of inserting a coupling member7 even in prostheses 1 of minimum thickness, while assuring a perfectand reproducible positioning of the stem-coupling member assembly,without any risks of a possible disengaging of the two parts.

FIGS. 13-15 show another possible embodiment in which, at the enlargedend of the stem 1, there is provided a lug 20 which has a substantiallytapering shape and an elliptical cross-section. From the lug 20 acylindrical portion 21 may project, the cylindrical portion 21 beingarranged on the axial extension of said lug 20. More specifically, thelug 20 can be engaged and firmly locked in a counter-shaped hollow 22,formed at the axial end portion of a coupling member 23 of theprosthesis. If desired the hollow 22 may be provided with a recess 24for housing the mentioned cylindrical portion 21.

One object of the invention is to reduce manufacturing costs byproviding differential hardness bearings surfaces in which each of theopposing articulating surfaces is manufactured from a cobalt chromealloy that is currently approved for orthopedic implant applications,thus reducing manufacturing steps. However, it would be possible toprovide the hardened component of differential bearing systems inalternative ways. The hardened component of a differential hardnessbearing system may be provided as a surface layer using techniques suchas vapor deposition of metal oxides or of ion bombardment to produce amixed metal-ceramic matrix at the surfaces. Alternatively, the layer maybe formed by heat treating such that a greater degree of hardness isconferred on the bearing surface than on underlying metal. The bearinglayer may either be discrete or gradually transition to the base metal.

Testing

This invention, including the unexpected results described below,resulted from wear testing that was initiated by Wright Medical with thegoal of finding the lowest wearing bearing coupling possible usingmaterials currently approved for orthopedic implant applications. Beforetesting differential hardness MOM bearings, Wright Medical tested COMbearing combinations to determine wear rates. As expected, Wrightobserved reduced wear rates for COM hip prostheses in comparison topublished MOM wear rates. Noting that COM hip prostheses typically havea hardness differential of about 4× between the ceramic and metalbearing surfaces, the inventor hypothesized that there may be a lowerlimit to the ability of differential hardness bearings to provideimproved wear performance. The limit of differential hardness inrelation to wear improvement has not been defined. Many wear models forhardness include only one value for hardness in the descriptionequation, and therefore cannot take into account the effect of usingbearings of different hardness.

In order to investigate the lower limit for hardness differentials,Wright Medical decided to test CoCr MOM hip protheses having arelatively low hardness differential between the head and shell. At thebeginning of testing, it was thought that CoCr MOM prostheses having arelatively low hardness differential might demonstrate slightly betterwear properties than existing MOM bearing combinations. In fact, asdiscussed below, testing yielded the unexpected result that hipprostheses manufactured from CoCr alloys having a hardness differentialproduce wear results comparable to those obtained with COM prostheses.

To test wear in the lower limit of differential hardness, wear testswere conducted on the following combinations of femoral head andacetabular cup materials:

-   -   1. Cast and thermally treated CoCr-Head and Cup (ASTM F75 for        both head and cup; Rc=25-30 for both head and cup; hardness        differential=1.0×; n=7)    -   2. Cast CoCr head/Cast CoCr cup (ASTM F75 for both head and cup;        Rc=25-30 for both head and cup; hardness differential=1.0×; n=3)    -   3. Wrought CoCr head/Cast-thermally treated CoCr cup (ASTM 1537        head/ASTM F75 cup; Rc=42 for head/25 for cup; hardness        differential=1.68×; n=3)    -   4. Heat treated wrought CoCr head/Cast-thermally treated CoCr        cup (ASTM F1537HT head/ASTM F75 cup; Rc=50-52 for head/25 for        cup; hardness differential=2.08×; n=2)

Wear testing was conducted to typical WMT wear test protocols for hipwear testing using a SHORE WESTERN OBM wear test machine (90% Bovineserum lubricant; triple-peak Paul profile (2000 N max @ 1 Hz); specimensin the inverted position (head above/shells below)). Specimens wereweighed at specific intervals to determine mass loss. All specimens were54 mm in diameter. The material combination described in group 1 aboveis currently used for Wright Medical's CONSERVE® PLUS MOM hipprosthesis. The combination of Group 1 produces low wear in comparisonto metal-on-polymer and competitive MOM prostheses. However, asmentioned above, some surgeons remain apprehensive about using MOMprostheses due to potential side effects from metal ion release fromwear debris, and there is thus an interest in further reducing wear.

In groups 3 and 4, Wright's typical CONSERVE® PLUS acetabular cups (thesame type of cups that were used in group 1) were tested against femoralheads made from wrought CoCr. The femoral heads of Group 3 weremanufactured from BIODUR CCM+MICROMELT™ CoCrMo bar (available fromCarpenter Technology of Reading, Pa.) in the as-received condition,which had a hardness of 42 Rc. The femoral heads of Group 4 weremanufactured from BIODUR CCM+MICROMELT™ CoCrMo bar in a heat treatedcondition (24 hours at 1350 degrees Fahrenheit in air), which resultedin a hardness of about 50 to 52 Rc. The wrought as-received bar and thewrought heat-treated bar provided a differential hardness when comparedto Wright Medical's typical acetabular cup (Rc=25). This difference inhardness was in the range of about 1.5 to 2×. As mentioned above, thehardness differential for ceramic-metal bearing combinations istypically on the order of 4× and can range from about 3× to 5× or evenhigher.

Test results are shown in the graphs of FIGS. 2 and 3. The test resultsof FIGS. 2 and 3 demonstrate several unexpected properties ofdifferential hardness MOM hip prostheses bearings: (1) significantlylower wear versus conventional MOM prostheses of the same type and size;(2) wear ratios that match those of COM prostheses when compared toconventional MOM prostheses of the same size and type; (3) overall wearmatching or exceeding overall wear for COM prostheses; and (4)elimination of a bedding-in phase.

As shown in FIG. 2, the differential hardness metal bearings exhibitedmuch lower wear than Wright's conventional CONSERVE® PLUS MOM femoralprosthesis. While a slight decrease in wear was expected for thedifferential hardness MOM bearings versus the conventional CONSERVE®PLUS MOM prosthesis, the magnitude of the reduction was surprising. TheGroup 3 MOM prostheses having a hardness differential of 1.68× showedonly about 0.107 mm³ of wear during the entire test. The Group 2 MOMprostheses having a hardness differential of 2.08 showed only about0.298 mm³ of wear during the entire test. In contrast, CONSERVE® PLUSMOM prosthesis having a conventional hardness differential of 1.0×demonstrated total wear of about 1.478 mm³ during the test. Thus, theGroup 3 CoCr MOM bearings demonstrated approximately 14× lower wear thanthe CONSERVE® PLUS MOM bearings of identical size (54 mm). The Group 4CoCr MOM bearings demonstrated approximately 5.0× lower wear than theconventional CONSERVE® PLUS MOM bearings of identical size (54 mm). Thetest results were also surprising in comparison with published data forMOM hip prostheses. For example, Firkins (discussed above), usingdifferent test methods, had shown MOM wear rates of about 8 mm³ afterfour million cycles.

Unexpectedly, when the differential hardness CoCr MOM bearings and COMbearings were compared to conventional MOM bearings of the same size andtype, the differential hardness CoCr MOM and the COM bearings exhibitedsimilar reductions in wear. FIG. 3 shows that COM bearings tested byWright Medical exhibited about a 14.5× reduction in wear as compared toa conventional MOM system of identical size (32 mm). As mentioned above,the Group 3 CoCr MOM bearings having a hardness differential of 1.68×demonstrated approximately 14.0× lower wear than typical CONSERVE® PLUSMOM bearings of identical size (54 mm).

In terms of overall wear, the wear of the Group 3 differential hardnessMOM bearings was on average lower than that of the COM bearings (0.107mm³ for differential MOM versus 0.172 mm for COM). Although the Group 4differential hardness MOM bearings did not match wear results for COMbearings, the overall wear of the Group 4 bearings was on the same orderof magnitude as COM bearings (0.298 mm³ for 2.08× differential MOMversus 0.172 mm³ for COM). This represented a significant reduction overthe conventional CONSERVE® PLUS MOM bearings, which showed almost a tenfold increase in wear versus COM bearings (1.478 mm³ versus 0.172 mm³).As mentioned above, the brittleness and lower toughness of ceramicmaterials make it difficult to manufacture large diameter ceramic heads(e.g. 54 mm) of the type used in hip resurfacing procedures. Theinvention solves this problem by allowing for the manufacture of hipprostheses that match the wear rates of COM bearings while essentiallyeliminating the risk of fracture associated with ceramics, a result thatcould have particular significance for large size hip prostheses.

As shown in FIG. 3, the differential hardness metal bearings alsoexhibited no bedding-in phase. This result was also unexpected because,as mentioned above, it was previously accepted that MOM prostheses havea bedding-in phase (see e.g. WO 0117464A1 reported a bedding in rate of3.12±0.45 mm³/10⁶ cycles; see also Firkins). As shown in FIG. 3,Wright's CONSERVE® PLUS MOM prosthesis exhibited a bedding-in wear phaseof about 3.1 mm³ While a slight reduction in bedding-in rate wasexpected for differential hardness MOM bearings, elimination of thebedding-in phase was a surprising result. Since the bedding-in phaseproduces in a large amount of metal particulates, elimination of thebedding-in phase is significant to patients who receive prosthesesimplants.

There are several reasons why the wear data presented in FIGS. 2 and 3is unexpected. The difference in hardness for the MOM bearingcombination was half that of the COM system, which suggested that wearrates would be significantly higher for differential hardness MOMbearings than for COM bearings. The MOM hardness differential wasachieved over a low hardness range (about 25-50 Rc for the MOMbearings), whereas COM prostheses achieve differential hardness over alarge hardness range. CoCr alloy is less abrasion resistant thanceramic. All of the foregoing factors suggested that wear rates shouldhave been significantly higher for differential hardness MOM bearingsthan for COM bearings, a result that would have been consistent withpublished results on MOM wear rates. As mentioned above, while a slightdecrease in wear was expected for the differential hardness bearings,the magnitude of the reduction was surprising.

Additionally, the prior art taught away from any suggestion that MOMprostheses, much less prosthesis bearings manufactured from alloyscurrently approved for use in prostheses, could achieve wear levelscomparable to COM prostheses. For example, Firkins reported wear ratesfor MOM prostheses that showed a 100 fold higher degree of wear than forCOM prostheses. Id. at 1296 (discussed above). There is no suggestion inFirkins or elsewhere that a MOM prosthesis could achieve wear levelscomparable to COM prostheses. Yet, surprisingly, the results reported inFIG. 2 are similar to Firkins' reported wear rates for COM hipprostheses. Id. at 1294. The differential hardness bearing concept ofthe present invention thus produces unexpected results.

Although the present invention has been described in terms of specificembodiments, it is anticipated that alterations and modificationsthereof will no doubt become apparent to those skilled in the art. It istherefore intended that the following claims be interpreted as coveringall alterations and modifications that fall within the true spirit andscope of the invention.

1. A hip prosthesis comprising: a spherical bearing member, saidspherical bearing member having a neck recess therein, a femoral stem,said femoral stem having a recess adjacent a proximal end thereof, amodular neck member, a proximal end of said modular neck memberconfigured to fixedly engage said neck recess of said spherical bearingmember, a distal end of said modular neck member configured to fixedlyengage said recess of said stem, an acetabular implant having a concavebearing surface configured for articulation with said spherical bearingmember, wherein one of said spherical bearing member and said concavebearing surface comprises a soft metal bearing surface of cobalt chromealloy, said soft metal bearing surface having a hardness of at leastabout 20 Rc, and wherein said other one of said spherical bearing memberand said concave bearing surface comprises a hard metal bearing surfaceof cobalt chrome alloy, said hard metal bearing surface having ahardness greater than said soft metal bearing surface by at least about15 Rc, such that there is a hardness differential between said sphericalmember and said concave bearing surface to thereby reduce wear of saidhip prosthesis.
 2. The prosthesis of claim 1, wherein the differentialhardness of said hard metal bearing surface to said soft metal bearingsurface is at least about 1.5.
 3. The prosthesis of claim 1, wherein thedifferential hardness of said hard metal bearing surface to said softmetal bearing surface is between about 1.5 and about
 3. 4. Theprosthesis of claim 1, wherein said hard metal bearing surface is notmore than about 40 Rc harder than said soft metal bearing surface. 5.The prosthesis of claim 1, wherein said hard metal bearing surface has ahardness of between about 40 Rc and about 60 Rc.
 6. The prosthesis ofclaim 1, wherein said spherical bearing member comprises said hard metalbearing surface and said concave bearing member comprises said softmetal bearing surface.
 7. The prosthesis of claim 1, wherein said cobaltchrome alloy of said soft metal bearing surface conforms to ASTM F75 andsaid cobalt chrome alloy of said hard metal bearing surface conforms toASTM
 1537. 8. The prosthesis of claim 1, wherein said hard metal bearingsurface is heat treated to increase hardness of said hard metal bearingsurface.
 9. The prosthesis of claim 1, wherein said spherical bearingmember has a diameter of between about 40 mm to about 60 mm.
 10. Theprosthesis of claim 1, wherein said spherical bearing member has adiameter of about 54 mm.
 11. The prosthesis of claim 4, wherein saidhard metal bearing surface has a hardness of between about 40 Rc andabout 60 Rc.
 12. The prosthesis of claim 11, wherein said sphericalbearing member comprises said hard metal bearing surface and saidconcave bearing member comprises said soft metal bearing surface. 13.The prosthesis of claim 12, wherein said cobalt chrome alloy of saidsoft metal bearing surface conforms to ASTM F75 and said cobalt chromealloy of said hard metal bearing surface conforms to ASTM
 1537. 14. Theprosthesis of claim 1, wherein said soft metal bearing surface has ahardness of between about 25 Rc to about 35 Rc.
 15. The prosthesis ofclaim 1, wherein said recess of said femoral stem is tapered.
 16. Theprosthesis of claim 15, wherein said recess of said femoral stem has anoval cross-section.
 17. The prosthesis of claim 15, wherein said recessof said femoral stem has a race track configuration in cross-section,said race track configuration having opposing flat sides and opposingcurved portions.
 18. The prosthesis of claim 15, wherein an axis of saiddistal end of said modular neck is coincident with an axis of saidrecess of said femoral stem.
 19. The prosthesis of claim 18, wherein anaxis of said proximal end of said modular neck is axially aligned withan axis of said distal end of said modular neck.
 20. The prosthesis ofclaim 18, wherein an axis of said proximal end of said modular neck isaxially offset from an axis of said distal end of said modular neck.